The present invention generally relates to printed wire board circuits and their fabrication. More particularly, this invention relates to a method for forming a thick-film resistor to have precise dimensions determined by photolithography techniques, thereby avoiding the variability associated with conventional screen printed resistors.
Thick-film resistors are employed in hybrid electronic circuits to provide a wide range of resistor values. Such resistors are formed by printing, such as screen printing, a thick-film resistive paste or ink on a substrate, which may be a printed wiring board (PWB), flexible circuit, or a ceramic or silicon substrate. Thick-film inks used with ceramic printed wire boards are typically composed of a glass frit composition, an electrically-conductive material, various additives used to favorably affect the final electrical properties of the resistor, and an organic vehicle or polymer matrix material. Thick-film inks used in organic printed wire board construction are typically composed of an electrically-conductive material, various additives used to favorably affect the final electrical properties of the resistor, an organic binder and an organic vehicle. After printing, the thick-film ink is typically heated to dry the ink and convert it into a suitable film that adheres to the substrate. If a polymer thick-film ink is used, the heating step serves to remove the organic vehicle and to cure the polymer matrix material. Other thick-film inks must be sintered, or fired, during which the ink is heated to burn off the organic vehicle and fuse the remaining solid material.
The electrical resistance of a thick-film resistor is dependent on the precision with which the resistor is produced, the stability of the resistor material, and the stability of the resistor tenninations. Control of the x, y and z dimensions (the width, electrical length and thickness, respectively, of the resistor) is particularly challenging in view of the techniques employed to print thick-film inks and the dimensional instability that may occur during subsequent processing. For rectangular screen-printed resistors, the x and z dimensions are determined by the screening process, and the y dimension is determined by the termination pattern. Conventional screen printing techniques generally employ a template with apertures bearing the positive image of the resistor to be created. The template, referred to as a screening mask, is placed above and in close proximity to the surface of the substrate on which the resistor is to be formed. The mask is then loaded with the resistive ink, and a squeegee blade is drawn across the surface of the mask to press the ink through the apertures and onto the surface of the substrate.
Compared to many other deposition processes, screen printing is a relatively crude process. As a result, screen printed thick-film resistors are typically limited to dimensions of larger than about one millimeter, with dimensional tolerances generally being larger than about .+-.10% at this lower limit. Consequently, screen printed thick-film resistors having adequate tolerances in the x and y dimensions are often larger than chip resistors. The thickness of a thickfilm resistor can generally be controlled to tolerances of about 20% to 30% by screen printing, due in large part to variability in the x, y and z dimensions. While the z dimension (thickness) of a screen-printed thick-film resistor can be reasonably well controlled through precision in the screening operation, the control of x and y dimensions is fundamentally limited by the relatively coarse mesh of the screen and by ink flow after deposition. As a result, resistance tolerances of less than .+-.20% cannot be achieved with screen printed thick-film resistors without laser trimming, an operation that is usually cost prohibitive for complex circuits.
From the above, it can be seen that present practices involving the fabrication of thick-film resistors can necessitate a compromise between the precision of the resistance value and the size of the resistor. In other words, while smaller resistors are often preferred to yield a more compact circuit, an undesirable consequence is that resistance values are less predictable due to the dimensional variability of the resistors. Accordingly, a need exists for a method for producing a thick-film resistor in which resistance values and tolerances can be more accurately controlled than prior art screen printing techniques permit.